The present invention relates to a photographic optical system and, more particularly, to a photographic optical system used in a camera for photography.
Photographic optical systems used in cameras for photography in recent years, particularly zoom optical systems are demanded to provide a high zoom ratio and to be compact in size. As the zoom ratio becomes higher, it becomes likelier that variations in aberrations, particularly variations in chromatic aberration, will limit the improvement in performance by aberration correction. Attempts have heretofore been made to improve chromatic aberration, for example, by using ED lenses.
ED lenses are low-refractive index and low-dispersive glass and have anomalous partial dispersion. Therefore, they allow removal of the secondary spectrum, which has heretofore been impossible to remove by the conventional optical glass alone.
Many examples are known as the prior art that uses ED lenses, including Japanese Patent Application Unexamined Publication (KOKAI) Nos. 8-160298, 4-95912, 3-260611 and 3-158816.
The above-mentioned prior art attains a compact zoom optical system by effectively using ED lenses in a lens arrangement comprising a minimal number of lens elements. Furthermore, the final lens unit is formed from a single negative lens having a concave surface directed toward the object side, thereby allowing the lens unit to come closer to the forward end of the lens system when it is collapsed. Accordingly, the arrangement is also favorable for achieving a reduction in the size of the optical system.
However, all these conventional zoom optical systems have a zoom ratio of 2 or less. Accordingly, the specifications of these zoom optical systems cannot be said to be attractive to users who demand zoom optical systems of high zoom ratio in recent years. Meanwhile, zoom optical systems designed to attain a high zoom ratio have an unfavorably large number of constituent lens elements, which is contrary to the purpose of attaining a compact optical system and likely to cause the cost to rise.
ED lenses are soft and readily scratchable in comparison to ordinary optical glasses and have a relatively large specific gravity (3.5 g/cm.sup.3 or more). Therefore, when ED lenses are used, it is necessary to consider that if a large number of ED lenses are used, the cost rises, and the optical system becomes heavy.